GREEN INFRASTRUCTURE DESIGNS BIOSWALE/HYBRID DITCH JULY 2015
BIOSWALE/HYBRID DITCH SCALABLE TOOL AND DESIGN TEMPLATE This tool and associated design files are one section of a larger toolkit: Green Infrastructure Designs: Scalable Solutions to Local Challenges that was developed with support from the National Oceanic and Atmospheric Administration through the Illinois Department of Natural Resources. Download Bioswale/Hybrid Ditch CAD files at ftp://deltaweb@www.delta-institute.org/cad/gi_bhd/gi_bhd.zip Download full toolkit Green Infrastructure Designs: Scalable Solutions to Local Challenges at www.delta-institute.org/tools. 2
BIOSWALE/HYBRID DITCH A bioswale / hybrid ditch functions in some ways like a conventional grass ditch that sits within the right-of-way along the road edge. Rainwater runoff flows across the crown of a road and enters at any point along its length. The main difference between the two is that a bioswale will have native plantings and a hybrid ditch will have grass. A bioswale / hybrid ditch slopes with the road to act as a conveyance channel which connects to an existing conventional ditch, surface water, or storm sewer. The significant difference between a bioswale / hybrid ditch and a conventional ditch is beneath the surface. A bioswale/ hybrid ditch has an engineered soil sub-base, which is a mixture of topsoil, sand, and compost. The importance of the engineered soil cannot be over-emphasized. It provides an effective filter for removing pollutants and sediment from runoff, and an excellent growing media for the native plants. A specification section for engineered soil that can be incorporated into the construction documents is located in Appendix C. In the case of the bioswale, a variety of native plants in the engineered soil take some of the runoff up through their dense root system. Additionally, an optional perforated underdrain within the stone layer can convey filtered water that does not infiltrate into the native soil. The overflow structure and perforated underdrain connect the stormwater planter to the larger storm network, carrying excess runoff downstream. If there are driveway crossings along the bioswale, then an overflow structure is set 6 to 9 inches above the engineered soil layer to convey stormwater from large rainfall events. The benefit of this technique is the scalable design length. The major drawback is the width required to achieve the appropriate storage depth, which can preclude its use or trigger the need for right-of-way acquisition depending on the site constraints. It does require maintenance and upkeep, the level of which depends on the type of vegetation that is planted on the surface. Location: Right-of-way Width: At least 5 Length: Scalable Drainage Area: <5 acres source: http://water.epa.gov/infrastructure/greeninfrastructure/gi_what.cfm 3
CUSTOMIZATION OPTIONS This technique can be customized in a number of different ways. The aesthetics can be tailored through native plant selection. It is recommended that native plant plugs are used to establish the bioswale. For a more mature plant aesthetic plants in gallons can be used, but this is more expensive. Hybrid ditches are typically planted with seed, but sod could be chosen for a higher quality finish at a premium price. Other customization options are a function of the site conditions. An overflow structure or culvert must be included at driveway crossings. An underdrain is required if the site is flat or is found to have poor infiltration for the native soils. Overflow pipes must be connected to the larger storm network either by connecting to an existing structure or by installing a new manhole atop an existing pipe. MAINTENANCE The native plants in bioswales have an establishment period and need to be watered 3 times a week for the first 4 weeks after installation. The native plants also need to be watered twice a week through October of the first year. After that point, the drought tolerant plants should withstand normal weather cycles. Other maintenance includes monthly debris removal, weeding, and pruning. The bioswales would also require a spring clean-up to remove built up debris from the winter, provide pre-emergent plant care and install / replace mulch. The perennials also need to be cut back in mid-march or November. A hybrid ditch is maintained like any other roadside ditch, requiring only regular mowing and debris removal. The maintenance costs for green infrastructure techniques with native plants and engineered soil is significant. If the installation is not maintained properly and on a regular basis, then the functionality of the system will become compromised. The specific cost will depend on the scale and complexity of the installation and the bidding environment for the labor contract. It is possible to self-perform the maintenance work or to save money by working with a notfor-profit. The design engineer should work to calculate a site- specific life cycle cost that accounts for maintenance when considering the feasibility of the project. COST INFORMATION Cost information is provided for each green infrastructure technique in Sections 5-9 of this report. The installed costs are based on project experience, bid tabs, and information from the RS Means Building Construction Costs Data (2012 edition), which is an industry standard compilation of unit costs for various construction activities. The costs in the table below can be used to scope a project, but a projectspecific cost estimate should be prepared by the design engineer that takes into account the project scale and complexity, material cost trends, and the labor and bidding environment. 4
SPECIFICATIONS As discussed in Section 3, specifications are an important component in the design of green infrastructure. Along with the construction documents, the design engineer should make site-specific customizations to the following sections of the standard specifications from the Illinois Urban Manual in order to have a full set of specifications for a bioswale or hybrid ditch. Other sections can be included on an as-needed basis. Further instructions on the use of specifications are included in Appendix B, and an engineered soil specification is included in Appendix C. Construction Specifications 2 - Clearing and Grubbing 5 - Pollution Control 6 - Seeding, Sprigging and Mulching 7 - Construction Surveys 8 - Mobilization and Demobilization 21 - Excavation 23 Earthfill 24 - Drainfill 25 - Rockfill 26 Topsoiling 44 - Corrugated Polyethylene Tubing 46 - Tile Drains 94 Contractor Quality Control 95 - Geotextile 707 - Digging, Transporting, Planting, and Establishment of Trees, Shrubs and Vines 752 - Stripping, Stockpiling, Site Preparation and Spreading Topsoil Material Specifications 521 Aggregates for Drainfill and Filters 548 Corrugated Polyethylene Tubing 592 Geotextile 804 Material for Topsoiling Appendix C Engineered Soil 5